Answer:
C. a frequency higher than the original frequency.
Step-by-step explanation:
when sound waves are emitted and there is a movement between the source of the sound wave and the observer, a phenomenon known as the Doppler effect occurs, Doppler effect is the change in the observed frequency of a wave when there is a movement between the source and the observer relative to the transmitting medium.
when the source of the sound waves is rapidly approaching a person, the wavelength of the waves become shorter with an increase in frequency and a higher pitch as the distance between the source and the observer decreases. For example take a man walking and trowing balls continuously to a boy. As the man gets closer, the boy would be catching more balls than when the man was farther away because the balls would be arriving at a faster rate than they were due to a decrease in distance between them.